We shall investigate the commensal relationships of intestinal microflora among themselves and with the host animal. The metabolic and physiological consequences of the enzyme tryptophanase will be examined in intestinal microbial species that form the enzyme, in intestinal species that do not form the enzyme, and in the host animal. Interactions will be studied in chemostats and in mice, both conventional and gnotobiotic. A possible relationship between tryptophanase and host tryptophan metabolism will be probed.